In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. One result of the forum's work is the High Speed Downlink Packet Access (HSPA). The High Speed Packet Access (HSPA) enhances the WCDMA specification with High Speed Downlink Packet Access (HSDPA) in the downlink and Enhanced Dedicated Channel (E-DCH) in the uplink. These new channels are designed to support IP based communication efficiently, providing enhanced end-user performance and increased system capacity. Though originally designed for interactive and background applications, they provide as good or even better performance for conversational services than the existing circuit switched (CS) bearers.
Concerning High Speed Downlink Packet Access (HSDPA) generally, see, e.g., 3GPP TS 25.435 V9.2.0 (2010-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iub Interface User Plane Protocols for Common Transport Channel Data Streams (Release 9), as well as 3GPP TS 25.306 V9.3.0 (2010-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UE Radio Access capabilities (Release 9), all of which are which are incorporated herein by reference in its entirety.
HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following: shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
In shared channel transmission, radio resources, like spreading code space and transmission power in the case of CDMA-based transmission, are shared between users using time multiplexing. A high speed-downlink shared channel is one example of shared channel transmission. One significant benefit of shared channel transmission is more efficient utilization of available code resources as compared to dedicated channels. Higher data rates may also be attained using higher order modulation, which is more bandwidth efficient than lower order modulation, when channel conditions are favorable.
The radio base station monitors for the channel quality (CQI) of the high-speed downlink shared channel (HS-DSCH) and manages a priority queue maintained at the radio base station. The base station's priority queue (PQ) stores data which is to be sent on the high-speed downlink shared channel (HS-DSCH) over the air interface to the mobile terminal. In addition, knowing from the monitor the carrier quality of the HS-DSCH, the base station sends to the control node messages which authorize the control node to send more HS-DSCH data frames to the radio base station.
The mobile terminal reports a channel quality indicator (CQI) to the radio base station in charge of the cell, which results in generation of capacity allocation control frames which are sent to the control node regularly and/or per need bases, e.g. at urgent transitions. The authorizing messages include a “capacity allocation” which can be expressed in various ways, such as in terms of either bitrate or credits, for example. For example, capacity allocation expressed in credits may refer to a number of MAC-d PDUs that the radio network controller (RNC) is allowed to transmit for the MAC-d flow. In response to these authorizing messages, the control node sends further HS-DSCH frames to the radio base station.
Thus, the data in the priority queues is sent from a control node to a radio base station in protocol data units (PDUs). A number of PDUs may be included in each high-speed downlink shared channel (HS-DSCH) data frame.
In WCDMA radio networks, the Radio Network Controller (RNC) is responsible for ensuring that data is available in the Radio Base Station (RBS) for transmission. To facilitate this, the RNC sends Radio Link Control (RLC) data via a Transport Network (TN) to the RBS. In state of the art solutions, the RNC controls the buffer fill level in the RBS by sending RLC PDU's to the RBS via a TN Frame Protocol (FP). While the RBS sends capacity allocations (CA) as input to this traffic shaping, it is still the RNC that controls how much data and at which rate this shall be sent to the RBS.
Since RLC is terminated in the RNC and in the user equipment unit (UE), any RLC data residing in an RBS queue is seen as delayed by the RLC entities involved in the data transmission. RLC timers are started when the data leaves the transmitting RLC entity and reset when an acknowledged by the receiving RLC entity has been received at the transmitting RLC entity. As a result of the delay, the RNC will (at poll timer expiry) poll data that has been transmitted from the RNC but is residing in the RBS queue and consequently not yet acknowledged by the UE. Thus if the amount of data in the RBS Priority Queue (PQ) is large and the data rate over the air interface (Uu) is low then the RLC entity in the RNC may will most likely poll the RLC entity in the UE even though no data has been lost. Consequently this will lead to unnecessary retransmissions of RLC PDU's or poll super field (POLL_SUFI's) as defined in 3GPP TS 25.322 V.9.2.0 (2010-06), Technical Specification Group Radio Access Network, Radio Link Control (RLC) protocol specification (Release 9), incorporated herein by reference.
Related to RLC control messages there is a general issue when short control packets and data packets of varying length is queued in one queue. A control packet may be very short and will not require much radio resources to be successfully transmitted, but may have to wait for the transmission of a large user data packet. This situation may occur even if there is no congestion in the radio interface, it occurs only because it takes a certain time to transmit a large data packet.
An even more serious problem is posed by RLC retransmissions. If these are delayed by data already buffered in the priority queue (PQ) then this can lead to multiple requests for the same data even though the previously retransmitted data is already in transit but buffered in the RBS PQ. In this situation with delayed retransmissions, the UE may have time to send an additional RLC status report before the initial retransmission even has been transmitted to the UE over the air interface leading to a situation where multiple copies of the same data will be sent both over the TN and Uu interfaces. This results in an inefficient use of the TN and air interface resources since the additional copies of retransmitted RLC PDUs do not contribute to the user experienced throughput since they will be discarded by the receiving RLC entity in the UE.
A solution such as Distributed Active Queue Management (D-AQM which works on the principle of avoiding a queue build up in the RNC and forwarding data to the RBS as soon as possible has been proposed. The D-AQM proposed system will result in queuing in the RBS and not in the RNC. This is in contrast to prior art WCDMA flow control which attempts to limit the queue length in the RBS in order to avoid spurious RLC retransmissions as described above. Therefore, the RLC Round Trip Time (RTT) can be expected to vary over a greater range for a TN relying on D-AQM as opposed to a TN utilizing classic flow control. The RLC protocol does not support dynamic adaptation to the current measured delay, therefore a solution to handle this highly varying delay would be to increase the configured RLC timers but this would result in poor performance due to excessive polling delays in certain scenarios such as short data transmissions when only the last RLC PDU (which carries the poll) is lost.
While D-AQM attempts to adjust the PQ buffer length in the RBS to the Uu rate, typically targeting a certain buffer length or dwell time, the Uu bit rate for services over radio varies within large bounds over short periods of time and situations with large PQs and low Uu rates will thus occur. In such cases it will take time for the RBS to shorten the PQ and adjust to the lower rate leading to an increased RLC RTT and subsequent delay of RLC polls, RLC control PDU's and retransmissions. This is in turn may lead to wasted bandwidth both in the TN and RN due to the unnecessary transmission of polls and multiple copies of RLC retransmissions.
Therefore, state of the art solutions such as classic HS-DSCH flow control with distributed buffers or a D-AQM solutions will all take longer than necessary to convey RLC SDU data (e.g. TCP data) to higher layer since these RLC SDUs will, if RLC retransmissions occur, be delayed longer than necessary due to being placed last in the RBS PQ.